In preparing laminates for use in printed circuits, sheets of fibers, e.g., glass paper, mat, cloth, and cellulose paper, asbestos mat, and the like, are saturated with a thermosetting resin, e.g., phenolic, epoxy, polyester, and the like, and then heated to partially cure the resin to a so-called "B-stage." These sheets are cut and stacked and the stacks are compressed and heated to fully cure the resin and provide a board-like structure reinforced with the sheets. The electrical requirements of the printed circuits require that dimensional tolerances, especially thickness, be maintained within specifications. Multilayer circuit boards, which comprise a plurality of thin laminates, bonded together into a unitary structure, especially require very close tolerances between layers so as to maintain uniform electrical properties.
Conventional pressing procedures produce laminates which are elliptical in shape. Typical tolerances obtained by conventional methods result in variations in thickness of 10-20%. Traditionally, close tolerance laminates have been produced by the wasteful practice of removing the center section of the laminate and and discarding the remainder.
A method has now been developed for preparing close tolerance laminates by applying a pressure gradient to the stacks of B-staged resin saturated sheets during consolidation. By way of illustration, according to this invention, with thickness of 0.0023" to 0.040", there is achieved a maximum average center to edge variation of .+-.0.00015" for thin laminates (0.005"-0.015") and .+-.0.00085 for thicker ones (0.031"-0.040"). The method depends on the use of contour pads to supply the pressure gradient.